Phase failure relays protect motors, equipment and personnel from damage or injury caused by open phase, reversed phase sequence, or low voltage in a three phase system. Models are available for 50 and 60 Hz with voltages up to 575 volts. Motor control switchboards are a common application.

Operation:
The contacts of the relay will close only when it senses normal conditions of three phase power at the proper phase sequence.
The relay contacts will remain in their normally open position (de-energized) when voltage with incorrect phase sequence is applied, one or more phases are open, or at undervoltage condition.